A cavity wall is a wall made of two separate layers of brick or block with a gap (the cavity) between them. The outer layer keeps the rain out. The gap stops moisture reaching the inner layer. Most UK houses built after 1920 have cavity walls.
How cavity walls work
The outer leaf (the outside wall you see) is usually brick. Rain hits it, soaks in slightly, then runs down the inside face of the outer brick. The cavity (typically 50-100mm wide) stops that water reaching the inner leaf. The inner leaf (usually brick or concrete block) carries the structural load and keeps the inside dry.
Metal wall ties span the cavity to hold the two leaves together. Water drains out through weep vents (small gaps in the mortar near ground level).
Cavity wall vs solid wall
Solid walls are a single thickness of brick or stone with no gap. Most UK houses built before 1920 have solid walls. They're typically 220mm thick (one brick length).
Cavity walls are thicker (260-300mm total) because of the cavity and the two separate leaves.
How to tell if you have cavity walls
Check the brickwork pattern
Look at the outside of your house. If all the bricks run lengthways (stretcher bond), it's almost certainly a cavity wall. If you see alternating long bricks and short bricks end-on (Flemish bond or English bond), it's a solid wall.
Measure the wall thickness
Measure the wall at a door or window reveal. Cavity walls are usually 260-300mm thick. Solid walls are 220mm (one brick) or 330mm (one-and-a-half bricks). If your wall is around 270mm, it's probably cavity.
Check the build date
Houses built after 1920 in England and Wales are usually cavity. Houses built before 1920 are usually solid. There are exceptions (some Victorian terraces have cavity walls, some 1930s cottages have solid stone walls), but the date is a good first guess.
Look at your EPC
Your Energy Performance Certificate (EPC) lists wall construction type. It will say "cavity wall" or "solid wall" and whether insulation is present.
Why the cavity matters for insulation
Cavity walls can be insulated by filling the gap with insulation (beads, wool, or foam). This is cheap (£800 to £1,500) and effective.
Solid walls can't be cavity-filled because there's no cavity. You need external or internal wall insulation instead, which costs much more (£4,000 to £18,000).
If you have cavity walls, cavity fill is usually the best first step for improving insulation. See our guide on whether cavity wall insulation is worth it.
Cavity width
The cavity is typically 50-75mm in houses built 1920-1980 and 75-100mm in houses built after 1980. Wider cavities are easier to fill with insulation and perform better once filled.
Some older cavity walls (1920s-1940s) have narrow or irregular cavities (40-50mm, sometimes partially filled with rubble or mortar droppings). These are harder to insulate and more prone to damp problems after filling.
Partial cavity fill
Some houses built in the 1990s have partial cavity insulation fitted during construction. The cavity is partially filled, leaving a residual gap for drainage. If your EPC says "partial cavity insulation", you can't top it up by drilling and filling. The cavity is already as full as it should be.
Sources
- Building Research Establishment (2020), "Cavity wall construction: performance and retrofit", BRE Trust